1. Field of the Invention
This invention relates to geophysical exploration in general and to means for identifying the geophone group number to which recording channel 1 is connected for use with a multichannel recording system employing the common depth point technique.
2. Discussion of the Prior Art
In Common Depth Point (CDP) geophysical surveying practice, a very large number of geophone groups, say 250-500, are laid along a line of survey. A lesser number of recording channels, say 60-120, are connected to a set of geophone groups that include a like number of observation stations. As the survey progresses, the recording channels are repeatedly advanced to new sets of geophone groups. To that end, a so-called CDP switch is employed. The switch consists of two parts: A stator that includes as many switch contacts as there are geophone groups, and a mating cursor having contacts equal in number to the number of recording channels. At the beginning of a line of survey, the cursor may be physically positioned with respect to the stator so that cursor contact 1, coupled to recording channel 1, engages stator contact 1 that is connected to geophone group 1. Similarly, assuming a 60 channel recording system and 240 geophone groups on the ground the last channel, 60, is connected to geophone group 60.
As the survey proceeds, the cursor is moved so that recording channel 1 is connected to geophone group 2 and recording channel 60 is coupled to geophone group 61. Subsequently, the cursor is again moved so that recording channels 1 and 60 are coupled to geophone groups 3 an 63 and so on. Thus in the exemplary system there are always 60 active geophone groups that are connected to recording channels, and 180 inactive groups that remain idle.
At each observation station, an acoustic source is actuated to produce seismic signals that are detected by the active geophone groups, received by the recording channels and recorded on a recording medium such as magnetic tape. Usually just before seismic data are to be recorded, a short "header" or label portion is recorded which provides factual information about instrument settings, area of operations, time of day etc. for each separate seismic data recording at each of a plurality of stations.
When the recorded data are to be processed, it is essential that the geophysicist knows precisely which channel is connected to which geophone group. Ordinarily that information was formerly provided by hand-written notes that were submitted by the field operating technician. Such notes are subjected to human error such as perhaps, an improper cursor setting.
It is the purpose of this invention to provide means for automatically annotating a recording medium, such as a magnetic tape, with the number of the geophone group that is actually connected to recording channel 1 at any point during the survey.